Differences in the spectral characteristics of white clover exposed to gaseous pollutants and acid mist



White clover plants (Trifolium repens L. cv ‘Grasslands Huia’) were exposed to combinations of gaseous pollutants and acid mists similar to those found in many parts of upland Britain. The gaseous pollutant treatments were (a) charcoal-filtered air (control), (b) SO2+ NO2 (c) O3 and (d) O3+ SO2+ NO2. The acid mist treatments were 6 mm per week of solutions at pHs of 2.5, 3.5, 4.5 and 5.6. After 12 weeks, canopy reflectance was measured at green, red, near infrared and middle infrared wavelengths using a portable radiometer. Distinct changes in the characteristic spectral reflectance of the clover canopies were noted. The effect of the gaseous pollutants was significant when examined by analysis of variance. The two treatments containing ozone showed particularly marked changes and were statistically separable from the control in a pairwise comparison; the sulphur and nitrogen dioxide treatment was not statistically separable from the control. The acid mists had no significant effect on canopy reflectance. Simple and 4-waveband vegetation indices showed positive linear relationships with shoot dry weight. The O3+SO2+NO2, in particular, and O3 treatments produced marked decreases in shoot dry weight and vegetation index. The green and near infrared wavebands were the best discriminators of change in canopy reflectance.